Rasterisation is performed within graphics rendering systems. Rasterisation, also known as scan conversion, is the process of determining which pixels belong to a graphic object. A graphic object may represent a text character, an image, or a shape such as a rectangle. Rasterisation is only concerned with which pixels are activated (drawn), and this depends on the outline of the graphics object only. The outline of a graphic object is often described by a polygon. If the outline contains curves such as Bezier splines, the curves are often converted to a series of straight line segments that approximate the curve.
The colour or appearance of pixels activated by a graphic object depends on factors such as the fill of the object, transparency of the object, and how the object is composited with objects below the object.
Rasterisation can be performed in many different ways. In addition, there are different rules that define the relationship between a pixel and a graphic object that must exist for the pixel to be activated. The different rules are known as pixel placement rules. There are two general pixel placement rules, each with slight variations: centre-intersect pixel placement rule and area-intersect pixel placement rule.
The centre-intersect pixel placement rule specifies that any pixel whose centre lies within the outlines of the graphic object is activated. The centre-intersect pixel placement rule is used by many graphics renderers, graphics libraries and page description languages (PDLs), including OpenGL, DirectX, GDI and XPS. The centre-intersect pixel placement may be implemented in software or hardware, can be executed quickly, and has the advantage that if an object is subdivided into a set of smaller objects, the smaller objects cause exactly the same pixels to be activated, with no gaps or overlaps.
The area-intersect pixel placement rule specifies that any pixel whose area intersects the graphic object is activated. The area-intersect pixel placement rule is more difficult to implement than the centre-intersect placement rule. The area-intersect pixel placement rule has the advantage that all objects will be rendered, no matter how small the objects are or the relationship of the objects with the underlying pixel grid. The area-intersect pixel placement rule is used by PostScript and Adobe® PDF, which are the page description languages (PDLs) most commonly used for printing.
Most existing graphics renderers contain rasterisers that rasterise exclusively using the centre-intersect pixel placement rule. It is often difficult or impossible to replace the centre-intersect rasteriser with an area-intersect rasteriser, which is disadvantageous when using existing graphics renderers for print rendering.
In one known method, a graphic object is first decomposed into a set of rectangles whose edges are aligned to the pixel grid. When the rectangles are rasterised using a centre-intersect rasteriser, the output pixels adhere to the area-intersect pixel placement rule. The disadvantage of such methods is that the graphic object must be pre-rasterised to generate the rectangles. The rectangles are then rasterised a second time, incurring a performance penalty.